Re-enable QWindowsMimeMetafile with Qt6

Re-enable QWindowsMimeMetafile with Qt6

Qt 6.5.0 has dropped support for the QWindowsMime class
in favor of a new QWindowsMimeConverter class. They say:
  "If you have implementations of QWindowsMime or QMacMime in Qt 5,
   then those will almost directly translate to the new APIs but
   require less boiler-plate code to register the converters with Qt."
This may be true, but in practice they break binary compatibility with
previous versions. If you used the QWindowsMime class until Qt 6.4 by
using boiler-plate code to register the converters with Qt, you now
need to modify the sources and recompile. Fantastic!